Help identify valve on solar water heat system

[voodoo18]

Hello,

Went up to the roof to inspect system after 5 years. I found this valve to have a small drip leak. The valve had a sticker, however it is faded from sun damage. Can I get some assistance with identification?

Thanks

 

[ENB]

Looks to me like a check valve.

 

[Rednecktek]

Back pressure regulating valve. It makes sure there is always a minimum amount of pressure in there so you don't pull a vacuum. At least that's what it looks like to me.